Flowing liquid laser

ABSTRACT

Stimulated emission of radiation (laser action) is produced in materials generally classed as dyes. These dyes may be dissolved in a liquid solution. A quantity of dye in a flowing liquid solution in a module is pumped or excited by a laser beam radiating in the ultraviolet region which is focused to a line with a cylindrical lens. A rectangular beam of such radiation is produced by a pulsed cross field nitrogen gas laser. The focused line which is transverse to the beam produced by the exciting laser, and transverse to the direction of the flowing dye lies near the surface of the dye material in the working region of the module, and is substantially as long as the cell is wide. The module being an integral unit with a cell, reservoir, and circulating means attached, the working region of which lies within an intensifying optical cavity which may be formed by a 100% reflecting mirror and a 10% mirror both perpendicular to the line of focus of the pumping radiation. The stimulated emission from the dye material is characterized by a short pulse width and little loss of energy between the two lasers. High pulse rates with dye circulation, and high conversion efficiencies of the dye when so pumped, are obtained. For frequency adjustment the optical cavity substitutes for the 100% mirror a grating or Littrow prism at the appropriate angle. Further spectral narrowing is obtained by inserting a tilted Fabry-Perot etalon in the cavity. By using such a frequency tuner and a plurality of dye materials which emit stimulated radiation over different portions of the spectrum, the present device can provide laser radiation for virtually the whole visible spectrum and into the infrared and ultraviolet.

atem [1 1 1 3, [451 July 10,1973

[ FLOWING LIQUID LASER [75] inventor: Robert F. Caristi, Stoneham, Mass.

[73] Assignee: Avco Corporation, Cincinnati, Ohio [22] Filed: Mar. 16,1972 [21] Appl. No.: 235,216

Primary Examiner-William L. Sikes Attorney-Charles M. Hogan et al.

[57] ABSTRACT Stimulated emission of radiation (laser action) isproduced in materials generally classed as dyes. These dyes may bedissolved in a liquid solution. A quantity of dye in a flowing liquidsolution in a module is pumped or excited by a laser beam radiating inthe ul- SUPPLY traviolet region which is focused to a line with acylindrical lens. A rectangular beam of such radiation is produced by apulsed cross field nitrogen gas laser. The focused line which istransverse to the beam produced by the exciting laser, and transverse tothe direction of the flowing dye lies near the surface of the dyematerial in the working region of the module, and is substantially aslong as the cell is wide. The module being an integral unit with a cell,reservoir, and circulating means attached, the working region of whichlies within an intensifying optical cavity which may be formed by a l00%reflecting mirror and a 10% mirror both perpendicular to the line offocus of the pumping radiation.

The stimulated emission from the dye material is characterized by ashort pulse width and little loss of energy between the two lasers. Highpulse rates with dye circulation, and high conversion efiiciencies ofthe dye when so pumped, are obtained. For frequency adjustment theoptical cavity substitutes for the 100% mirror a grating or Littrowprism at the appropriate angle. Further spectral narrowing is obtainedby inserting a tilted Fabry- Perot etalon in the cavity. By using such afrequency tuner and a plurality of dye materials which emit stimulatedradiation over different portions of the spectrum, the present devicecan provide laser radiation for virtually the whole visible spectrum andinto the infrared and ultraviolet.

5 Claims, 2 Drawing Figures 1 rLowiNc noun) LASER BACKGROUND OF THEINVENTION The field of this invention relates to the production ofstimulated radiation in materials which are dyes or which haveproperties similar to those of dyes, and to methods and apparatus forproducing such stimulated radiation.

It has been known for a number of years that stimulated emission can beproduced in various organic liquids. The first such liquids were dyes,as reported by Sorokin et al., IBM Journal, Volume II, page 130, March1967, and since that time devices which have been used to produce suchstimulated radiation have been commonly known as dye lasers even thoughthe materials emitting the radiation could not be classified as dyes inthe true definition of the word. Some materials which fluoresce orscintillate outside the visible spectrum have been used, for example. Acompendium of materials which have served as the active medium in dyelasers is given both in the above cited article of Sorokin et al, and inthe review of Kagan et al., Laser Focus, page 26, September 1968.Because the term dye laser has become commonplace, it is used herein,but with the understanding that the active laser medium can be otherthan a dye.

The characteristics of dye lasers which make them attractive are thepossibility of wide spectral range and tunability at low cost. One canoperate the laser anywhere in the visible or into the ultraviolet orinfrared simply by using a solution which emits in the desired spectralregion in conjunction with an appropriate optical cavity. The cost ofthe material is minimal, certainly far less than the cost of a group ofconventional lasers emitting at different wavelengths, and also lessthan the cost of frequency doublers and other such devices. The outputwavelength of a dye laser also is tunable, either by varying theconcentration of the solvent or by introducing a wavelength selectiveelement such as a grating into the optical cavity to control theemission wavelength. Significant spectral narrowing without significantenergy reduction is an additional benefit obtained with the use of agrating. Line widths less than 1 angstrom can be obtained in contrast tothe 50-200 angstroms which are typical of dye laser emission.

Typical dye lasers used in an effort to obtain these characteristicshave been pumped with Q-switched ruby or glass lasers, frequencydoubled, or as in a few cases, pumping has been accomplished with flashlamps. Pumping has been either in a longitudinal geometry, in which thepumping radiation is colinear with the optical cavity axis andstimulated radiation, or in a transverse geometry with the excitation atright angles to this axis.

Dye lasers have thus far fallen short of achieving their full potential,however, because (I) a number of useful materials are difficult to pumpdue to low quantum efficiency or high excited state losses due tosinglet-triplet transitions or to triplet absorptions, (2) lowconversion efficiencies, high coupling energy losses, (3) low pulserepetition rates due to thermal effects induced during pumping, and (4)dye circulation problems and limitations. Therefore, the object of thepresent invention is to provide a dye laser capable to producingstimulated emission over a broad spectrum at high repetition rates, withfrequency tuning, and which is an economical and practical device withhigh pumping efficiency and low energy coupling loss.

It is another object of the invention to provide apparatus for and amethod of producing stimulated emission in a dye laser at highrepetition rates with more efficient operation.

A further object of the invention is to provide a homogeneous flowingdye medium which will maintain good optical properties from pulse topulse.

A still further object of the invention is to provide apparatus forallowing the dye solution to flow rapidly through the optical cavity ina nearly laminar or nonturbulent manner.

A still further object of the invention is to provide a self-containedflowing dye module comprising a cell, reservoir, and pumping systemwhich may be easily removed and replaced for ease of operation andmaintenance.

A still further object of the invention is to provide a flowing dye cellwhich substantially reduces the amount of undissolved contaminants fromflowing through the active working region of the laser.

A still further object of the invention is to provide a bypass channelin the flowing dye cell in order to eliminate air pockets or bubbles andthereby prevent them from flowing through the working region of thelaser.

According to the invention these objects are achieved in a dye laserwhich has a laser pump emitting a pulsed rectangular beam of exciting orpumping radiation, which beam is focused to a line by a cylindrical lensor mirror. The focus line of pumping radiation is directed to lie withina segregated quantity or working region of lasing material, which can bean organic dye or similar material. Optical cavity means to intensifystimulated radiation emitted along the focused line are provided with areflecting axis perpendicular to the pumping radiation. The opticalcavity means includes wavelength selective means such as a grating totune the output frequency. In preferred embodiment, the laser pump is acrossed field gas laser, more particularly a nitrogen gas laser emittingat 3,371 angstroms, the dye cell is substantially as wide as the focusedline of pumping radiation and the focused line of pumping radiation ispositioned near the inner surface of the working region of the cell. Thedye cell is of a U-shaped hy' drodynamic design which provides asubstantially laminar dye solution following through the working regionlocated at the bight. Further, the cell is designed to preventundissolved particles and air pockets to pass through the working regionwhich would cause deleterious effects. Further, the cell is an integralpart of the module which is an integrated plug-in type comprising acell, liquid pump, and reservoir for ease of operation and maintenance.

The novel features that are considered characteristic of the inventionare set forth in the appended claims; the invention itself, however,both as to its organization and method of operation, together withadditional objects and advantages thereof, will best be understood fromthe following description of a specific embodiment when read inconjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view with parts broken away of apparatus inaccordance with the invention; and

FIG. 2 is a detailed expanded perspective view with parts broken away ofa portion of apparatus illustrated in FIG. 1.

Attention is now directed to FIGS. 1 and 2 which illustrate a preferredembodiment of the invention. A

pumping laser 11 emits a rectangular beam 12 of coherent, collimatedpumping radiation in pulse form. The rectangular beam 12 is converged inone dimension as shown by a cylindrical lens 13 and is focused to a line14 of pumping radiation which is at right angles to the direction ofbeam 12. The working region of the cell, approximately as wide as theline 14, is positioned so that the line 14 lies within it to bring aboutthe excitation to higher energy levels and the necessary populationinversion for stimulated emission to occur in the laser material. Asimple optical cavity for intensification of stimulated radiation in thelaser material is formed by a conventional grating 20 and a planarmirror 21, both of which are perpendicular to line 14. The stimulatedradiation, at a wavelength determined by the laser material in the dyemodule and the grating passes through the mirror 21 as the output beam60 of the dye laser.

PUMPING LASER The source of pumping radiation in the present inventionis the pumping laser 11 which preferably is a pulsed cross field gaslaser either using nitrogen (N as the discharge gas and emitting at3,371 angstroms in the ultraviolet or using neon as the discharge gasand emitting at 5,401 angstroms in the green. A suitable example of sucha laser is that invented by R. Caristi, et al. described in US. Pat. No.3,633,127, issued Jan. 4, 1972. A commercial version of this laser isthe Avco Model C5000 pulsed gas laser.

The pumping laser 11 of the above type, as described more fully in theabove US. patent, is constructed with a U-shaped aluminum base electrode31, two spaced Lucite side walls 34 and 36 facing each other within thebase electrode, and an aluminum electrode 37 carried by the side walls34 and 36. Through the rectangular duct 39 formed between electrodes 31and 37 between side walls 34 and 36, a supply of gas is flowed atappropriate pressure, and a pulsed electric field is applied acrosselectrodes 31 and 37 to cause the pulsed rectangular laser beam 12 to beemitted. For purpose of the present invention the gas flowed throughduct 39 is preferably nitrogen, which produces radiation at 3,371angstroms in the ultraviolet for efficient pumping of most lasermaterials in the dye cell 15, but it can also be neon, which emits at5,401 angstroms in the green and is suitable for pumping of somematerials, specifically materials which emit in the near infrared.Further details of operation of pumping laser 11 appear in the patentmentioned above. The following table summarizes typical performancecharacteristics of the pumping laser 11 of the above type, employingnitrogen as the discharge gas.

Nitrogen Pumping Laser Characteristics Output wavelength A 337 lABandwidth AA 1A Peak output power l00kW Effective pulse widths l0nanoseconds Energy per pulse 0.00] joules Output beam dimension A" X 2"Pulse repetition rate 0 to 500 pps The high peak power and rapid pulserise time, as well as the high possible repetition rate, all contributeto the excellent results of use of such a device as a source of pumpingradiation.

Further focusing aspects of the pumping arrangement are shown in FIO. 1.FIG. 1 illustrates how the parallel waves of the rectangular beam 12 areconverged into the line 14 by the cylindrical lens 13. The position ofthe line of focus 14 within the working region dye cell 15 can be variedto suit the material in the dye cell 15. For materials which lase onlywith difficulty due to low quantum efficiency or excited state losses,highly concentrated pumping energy is desirable. Because many of theselaser materials in the dye cell 15 that are difficult to excite have ashort absorption length in the concentrations which produce optimumpower, it has been found preferable for these materials to locate thefocus line 14 near the surface of the laser material'so there will belittle absorption loss to reduce the intense concentration of pumpingenergy potentially available at focus line 14. Typical distances betweenthe line 14 and the surface of such lasing materials have been on theorder of a few 'hundredths to V a few tenths of a millimeter. Formaterials which lase readily, highly concentrated pumping energy mayproduce the undesirable condition of super-radiance, in which radiationis emitted uncontrolled by the optical cavity. For such materials thefocus line 14 is located further from the surface of the material sothat the energy concentration in the active medium is reduced by anincrease in lasing volume.

DYE MODULE Attention is directed to FIGS. 1 and 2 which illustrate theconstruction of dye module 80. The dye module is designed to hold andcirculate a liquid laser material and comprises a rectangular cellcontainer generally designated 41, a reservoir 42, and a circulatingliquid pump 43. The components, container 41, reservoir 42, and pump 43are fixedly mounted on plate 44. Plate 44 is provided with twopositioning pins 45 and 46 and quick release bolt 47 for positioning andattaching the dye module on to the supporting structure 48. The modulemay be positioned in the proper relation to the optical cavity and laserpumping means by attaching plate 44 to structure 48. This design allowsthe dye module 80 to be easily inserted into the system and facilitatesoperation and maintenance. For example if another dye solution isdesired, the whole dye module may be readily detached by merely turningbolt 47, removed, and replaced by another module which is positioned bypins 45 and 46 that are permanently affixed to structure 48. Thus, it issimilar to the operation of removing and replacing a cassette in a taperecorder.

In operation the liquid dye solution is drawn from container 41 by pump43 and pumped into the tilted elevated reservoir 42. The dye solution isintroduced into the reservoir 42 from above the surface of the solutionby a fill tube 50 positioned inside the reservoir. The fill tube ispositioned in such a manner as to introduce the solution along the lowerside of the inclined reservoir. The solution is introduced in such amanner to avoid the froth or bubbles much like avoiding foam when beeris poured into a tilted glass. The reservoir 42 is elevated in order toprovide a gravity assist when the dye solution is introduced to theinlet manifold 58 of container 41. Both inlet and outlet manifolds (58and 66 respectively) comprise a cylindrical channel 52 which receivesand stores a quantity of the dye solution. Along one side of the channel52 is arranged a number of small passages which in actual practice, due

to the pressure provided by the pump 43, function like nozzles,collectively identified by the numeral 53. In operation, the inletmanifold 58 receives the solution from the reservoir 42 and supplies thesolution to the inlet duct 51 through the nozzles 53. This arrangementbreaks up the flow into a number of small jet streams emerging from eachof the nozzles in order to minimize turbulence and thereby reduce bubbleproduction. After the solution enters the rectangular U-shaped flowchannel through inlet duct 51, it passes through a screen or reticulatedstructure to further smooth the flow as well as remove solid undissolvedparticles and restrict the flow or air bubbles through the system. Anescape slot 55 is provided to allow the bubbles which have accumulatedat the elevated side of the flow channel to escape without circulatingthrough the working region 56. The cross section of the flow channel isreduced at the working region 56 of the cell located at the bight of theU-shaped channel. At the working region 56 the flow velocity should bemaximum and the motion nearly laminar. This region should also be freefrom any bubbles or solid particles which would interfere with thelasing action. As previously mentioned, the pumping laser 11 is focusedto a line 14 in the working region 56 in order to create the populationinversion within the dye solution. The focused pumping or exciting laserbeam 12 is introduced through the front window 70 substantiallytransverse to the flow direction as well as transverse to the dye laseroutput beam 60. In order to minimize the effects of output windows 62and 63 on the optical cavity, the whole module is canted at an angle 61which is only a few degrees. This canting angle is effective to tilt theplane of the windows 62 and 63 in order to form an obtuse angle betweenthe output window and the output beam 60.

After the dye solution passes through the working region 56, the dyesolution then passes through the outlet flow duct 65, and outletmanifold 66. The dye solution is then drawn out of the exit manifold 66of the container 41 by a pump 43, pumped into the reservoir 42, andreturned to the container 41 through inlet manifold 58 to complete thecycle. The cycle may be repeated until the dye solution breaks down. Therepetition rate is increased because the solution is continually beingcirculated providing a fresh amount of dye solution within the workingregion. This fresh solution is capable of being excited and is free fromany residual stored energy (such as heat or populated energy states)which the previously excited segment of the dye solution would contain.

OPTICAL CAVITY The radiation stimulated in the laser material in theworking region of the dye module is intensified by an optical cavityhaving its reflecting axis parallel to line 14 and transverse to thedirection of the pumping beam 12. As mentioned above and as shown inFIG. 1, the optical cavity can be comprised of a grating 20 and apartially reflective mirror 21 or alternatively may be comprised of twomirrors, one of which is 100% reflecting for greatest intensification(replacing the reflective grating), and the other of which is partiallytransmitting (e.g. reflecting) to permit output beam 60 if stimulatedradiation to exit from the device.

As shown in FIG. 1, the grating reflector may be rotated and therebyused to provide wavelength tuning. The criteria to be followed inselecting the properties of grating 20 will depend to some extent on theuse to which the output beam 60 is to be put, but it is generallydesirable to select a grating which is efficient at the wavelengths ofinterest and with blaze selected for highest energy, and with highresolution and dispersion to produce an output of greatestmonochromaticity. The relationships of groove spacing and number, andblaze, to produce these results are well known and need not be repeatedhere. Tuning is achieved, as is well known, by varying the angle of thegrating until the desired wavelength is obtained. Other wavelengthselective elements, for example a Littrow prism, can also be substitutedfor grating 20.

For further spectral narrowing a transmission filter (not shown) can beinserted in the optical cavity. In such a situation the use of aFabry-Perot etalon is positioned at an appropriate angle to the cavityaxis to pass radiation of the desired frequency. The tilted partiallyreflecting inner surfaces of this etalon form a resonant cavity whichprovides spectral narrowing to a high de' gree, and line widths on theorder of 0.01 angstroms in output beam are obtainable with such adevice. Furthermore the grating in the optical cavity can be replacedwith a concave mirror if an increase in cavity stability is desired.Further the grating can also be replaced with a convex mirror to form anunstable cavity which will help to suppress any unwanted modes.

OPERATION AND RESULTS The dye laser arranged as described above has beentested with a variety of laser materials. It was found that highconversion efficiencies could be obtained. Moreover, the extremely shortpulse widths and the fast repetition rate with the use of dyecirculation which were obtained are two unique and valuable features ofthe present scheme. Use of a diffraction grating instead of a reflectingmirror for one end of the optical cavity permitted the laser wavelengthto be tuned and also permitted the effective wavelength range of eachdye to be extended. Tuning of the radiation to a narrow band widthresulted in no apparent loss of efficiencies at the wavelengths of thenatural fluorescence, but there was observed a definite variation ofefflciency with wavelength. Efficiency is also a function ofconcentration of the dye, and the output spectral distribu tion is alsoa function of concentration. The rapid pulse rate with little loss inefficiency which is obtainable in the present invention indicates thatthe recovery of the dyes is fast when pumped in the manner describedabove.

Typical test results using the dye laser of the present invention aresummarized in the table below. The concentrations of the various dyeswere selected for maximum conversion efficiency, the concentrationsgenerally being on the order of 10 to 10 moles/per litre. Conversionefficiency given in the table is the ratio of the dye energy to thenitrogen laser energy, i.e. the energy of the dye laser pulses dividedby the energy of the nitrogen laser pulses.

TYPICAL CHARACTERISTICS OF FLOWING DYE LASER EXCITED BY PULSED NITROGENLASER (3371 ANGSTROMS) Tunable Pulse Conversion Pulse Range EnergyEfficiency Width Dye/Solvent (A) (Micro- (Percent) (Nsec) joules) PBD/3550-3860 47 4.7 S butyl alcohol As the above table makes clear, thepresent dye laser combines high efficiency with short pulse widths andrapid pulse rates to make this a very valuable method of stimulatingradiation in dyes. By providing a variety of different dyes in differentinterchangeable dye modules, it can be appreciated that the present dyelaser will permit a broad spectral range to be covered at highefficiencies simply by replacing dye modules and by making simpleadjustments in the grating.

The various features and advantages of the invention are thought to beclear from the foregoing description. Various other features andadvantages not specifically enumerated will undoubtedly occur to thosversed in the art as likewise will many variations and modifications ofthe preferred embodiment illustrated, all of which may be achievedwithout departing from the spirit and scope of the invention defined bythe following claims.

What is claimed is:

1. Apparatus for producing stimulated radiation in a flowing liquidlaser material comprising:

a. pumping laser means emitting a rectangular beam of pumping radiation;

b. focusing means positioned to focus said rectangular beamsubstantially to a line;

c. container means including means defining a U- shaped channel having arestricted portion at its bight for providing substantially laminar flowof said laser material through a restricted area, said restrictedportion defining the working region wherein the flow velocity isincreased, said container having a front window through which saidfocused beam passes and said container being positioned such that saidline of focus is substantially transverse to said flow direction and islocated within said working region, said container additionally havingtwo side windows through which the stimulated emission produced in theworking re gion may pass, said container means including an inletmanifold and substantially rectangular inlet duct means through whichsaid liquid laser material flows to said working region, and an outletmanifold and substantially rectangular outlet duct means through whichsaid liquid laser material flows after leaving said working region, saidmanifold means each having a chamber portion and having a plurality ofpassages spaced along the length of said chamber portion providingcommunication between the interior of respectively said inlet and outletchamber portions and saidinlet and outlet duct means, whereby said lasermaterial flow is substantially laminar on entering and leaving saidworking region;

d. liquid pumping means circulating said liquid laser material throughsaid container; and

e. optical cavity means spaced from said container for intensifyingstimulated radiation emitted from said liquid laser materialsubstantially along said line of focus, and optical cavity means havinga reflecting axis substantially coincident with said line of focus ofpumping radiation.

2. Apparatus according to claim 1 wherein said liquid pumping meansincludes a reservoir disposed at least in part above said containermeans to hydrostatically assist said flow.

3. Apparatus according to claim 2 wherein said reservoir is tilted at anangle, said reservoir being filled from said container means through afill tube positioned so as to introduce said laser material along thelower inner inclined surface of said reservoir.

4. Apparatus according to claim 1 wherein said inlet duct means islocated below said outlet duct means and said inlet duct means includesa reticulated structure positioned transverse to said flow direction andsubstantially covering the cross sectional area of said inlet duct,whereby said laser material passes through said reticulated structureand is filtered prior to entering said working region.

5. Apparatus according to claim 4 wherein said duct means includes aslot upstream and adjacent said reticulated structure, said slotinterconnecting said inlet and outlet duct to provide a relief passagefor air bubbles trapped by said reticulated structure.

2. Apparatus according to claim 1 wherein said liquid pumping meansincludes a reservoir disposed at least in part above said containermeans to hydrostatically assist said flow.
 3. Apparatus according toclaim 2 wherein said reservoir is tilted at an angle, said reservoirbeing filled from said container means through a fill tube positioned soas to introduce said laser material along the lower inner inclinedsurface of said reservoir.
 4. Apparatus according to claim 1 whereinsaid inlet duct means is located below said outlet duct means and saidinlet duct means includes a reticulated structure positioned transverseto said flow direction and substantially covering the cross sectionalarea of said inlet duct, whereby said laser material passes through saidreticulated structure and is filtered prior to entering said workingregion.
 5. Apparatus according to claim 4 wherein said duct meansincludes a slot upstream and adjacent said reticulated structure, saiDslot interconnecting said inlet and outlet duct to provide a reliefpassage for air bubbles trapped by said reticulated structure.